This relates to an improved control and control method for a paving machine and, more particularly, to such a control and control method for paving machines that utilized floating screeds for contouring the surface of the paved area to match adjacent, reference surfaces such as adjacent paved areas, adjacent curbs, and adjacent string lines that extend between surveyor located stakes.
Paving machines of this type typically include a tractor or towing vehicle that moves along a subgrade to be paved. The paving machine deposits a layer of asphalt or other paving material on the subgrade, and the thickness and contour of the asphalt layer are determined by a floating screed that is towed behind the vehicle. The screed has a plate that rides over the surface of the asphalt behind the vehicle, and a pair of forward extending tow arms. The tow arms are connected to the vehicle at tow points which can be raised and lowered by hydraulic cylinders. When the tow points are raised, the front edge of the screed plate is raised and the orientation of the plate is such that it tends to plane upward over the asphalt that is being deposited on the subgrade just ahead of the screed, raising the top surface of the asphalt layer. Conversely, when the tow points are lowered, the front edge of the screed plate is also lowered, reorienting the plate to plane downward, and lowering the top surface of the asphalt layer. It will be appreciated, that the screed smoothes the top surface of the layer of the paving material, while at the same time controlling the vertical position of this surface.
The paving machine deposits the paving material on the subgrade so that the top surface of the paving material follows a desired elevation contour. For example, when a second strip of asphalt is deposited on a roadbed next to a first strip of asphalt, it is desired that the surface height of the two strips match precisely at the seam where they abut. As another example, when an asphalt layer is deposited on a subgrade next to an existing street curb, it may be desired that the asphalt surface height be controlled precisely with respect to the curb. As a further example, a surveyor may have previously surveyed a road or other surface to be paved and set a series of stakes with a reference string line running from the top of one stake to the top of the next stake. In each instance, it is necessary that the vertical position of the top surface of the deposited paving material be controlled precisely with respect to a reference surface of some sort, and this requires that the tow points of the tow arms be controlled with precision.
Prior art paving machines have used a number of approaches to controlling the screed. It has been common to attach a horizontal bar to the tow point so that the bar extends parallel to the direction of movement of the paving machine and laterally displaced to the side of the machine. The bar is mechanically constrained in a horizontal position and has a number of downward facing ultrasonic sensors mounted along its length. The sensors determine the distances to the reference surface at measurement points directly beneath them. These distances can then be used to control the raising and lowering of the tow point. The tow point on the opposite side of the machine may be raised and lowered by the same amount, or it may be controlled independently using a second horizontal bar and a second set of ultrasonic sensors.
Such prior art control arrangements for paving machines are cumbersome, difficult to transport, and easily damaged. An improved control and method are needed for paving machines having fewer sensors and less cumbersome mounting arrangements, and providing effective control of the screed.